This invention relates to a method of producing waveforms of desired shape and more particularly to a system for obtaining output waveforms of selected shape through the transformation of readily available optical input pulses.
There are many applications for systems capable of producing specific optical or electrical waveforms. Generally, in the design and testing of optical or electro-optical devices, it is necessary to measure the ability of the device to follow the input optical waveform or alternately to measure the distortion in the waveform introduced by the device. Since it is often necessary to test the response of the device to a range of input optical waveforms (e.g., various rise and decay times), a waveform generator capable of variable outputs is particularly desirable. Similarly, electrical waveforms are utilized to measure the response of circuits to known inputs. In particular, in nuclear explosives diagnostics where pulse shapes are well documented but not normally available, both simulated electrical and optical waveforms are valuable in testing circuitry as well as data handling systems.
Prior techniques for synthesizing pulses of desired shape are presently quite limited, particularly for pulse widths less than about 1 usec. Prior techniques for generating specially shaped electrical pulses have generally used resistive-capacitive networks which use diodes, transistors, or other active elements. These generally produce pulse shapes which have undesirable variations in rate of voltage change, have limited dynamic range, and are not highly reproducible. Transmission lines have also been used to shape pulses, primarily by storing energy in a coaxial line which is switched into a load by a fast switch. Pulses produced in this way are frequently accompanied by additional unwanted pulses caused by unavoidable reflections at the transmission line impedance mismatches. Synthesis of electrical pulse shapes using multiple electrical source generators is also generally unsatisfactory due to the impedance matching problem.
Optical pulses have frequently been shaped by the use of mechanical shutters, rotating apertures, or mirrors. However, these approaches have not been very successful in shaping pulses with pulse widths less than about 1 usec. Optical pulse shaping techniques in the 1 -10 ns range have been limited mainly to electro-optical devices whose transfer functions are either fixed or confined to a narrow range. Some shaping of the output of optical sources, such as lasers and flash lamps, is possible through alterations in the energy transfer circuitry, but this is limited to rather narrow regions .
The invention is a method by which an optical pulse of an arbitrary but defined shape may be transformed into a virtual multitude of optical or electrical output pulse shapes. Since the method is not limited to any particular input pulse shape, the output pulse shapes that can be generated thereby are virtually unlimited. Moreover, output pulse widths as narrow as about 0.1 nsec can be readily obtained since optical pulses of less than a few picoseconds are available for use as driving pulses. The range of output pulse widths obtainable is very large, the limiting factors being the driving source energy and the particular shape of the desired output pulse.